bad battery fry starter

[Anonymous]

Here is a query for the electrical engineers in the group. An old starter fixer guy told me that the worse thing for a starter is to run it on a battery with one or more cells dead. The explanation for this was that since each dead cell represents a 2 volt reduction in emf (electromotive force). He said that each electric motor will operate at a given wattage reguardless of the voltage supplied. Since V x A = W, if the voltage supplied by the battery drops the motor will draw more amps to maintain the wattage. More amps means more heat since it is the current flow (amps) and not the force (volts) that produces heat. Heat builds up and the starter fries (Would you like catsup or vinegar on those fries?) sooner. Since the voltage is low from the dead cells, the motor will not turn over as well and be less likely to start as easily. The starter will be applied for a longer period and the heat will build up. Starter motors are not good at getting rid of heat because they are not designed to operate for more than a few seconds at a time. So, my question is: Does this sound valid from an engineering perspective?

Junkman

 

[Hap Call]

Re: bad battery fry starter

Here is a query for the electrical engineers in the group. An old starter fixer guy told me that the worse thing for a starter is to run it on a battery with one or more cells dead. The explanation for this was that since each dead cell represents a 2 volt reduction in emf (electromotive force). He said that each electric motor will operate at a given wattage reguardless of the voltage supplied. Since V x A = W, if the voltage supplied by the battery drops the motor will draw more amps to maintain the wattage. More amps means more heat since it is the current flow (amps) and not the force (volts) that produces heat. Heat builds up and the starter fries (Would you like catsup or vinegar on those fries?) sooner. Since the voltage is low from the dead cells, the motor will not turn over as well and be less likely to start as easily. The starter will be applied for a longer period and the heat will build up. Starter motors are not good at getting rid of heat because they are not designed to operate for more than a few seconds at a time. So, my question is: Does this sound valid from an engineering perspective?

Junkman

Yes

Hap

 

[Anonymous]

Sometime you are too deep and technical in your explanations Hap :lol: Can you say that in simpler terms please 8O

 

[Hap Call]

Sometime you are too deep and technical in your explanations Hap :lol: Can you say that in simpler terms please 8O

No

Hap

 

[Anonymous]

Sometime you are too deep and technical in your explanations Hap :lol: Can you say that in simpler terms please 8O

No

Hap

:lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol:

 

[focus frenzy]

as Hap said, yes, he is correct, lower the voltage and the amps will go up, causing the starter to heat up quicker.

 

[Anonymous]

and an excessive voltage drop can cause enough current to be drawn to melt some of the windings, MMMMMM Smell good 8O 8O

 

[Hap Call]

and an excessive voltage drop can cause enough current to be drawn to melt some of the windings, MMMMMM Smell good 8O 8O

Maybe

Hap

 

[Anonymous]

bad battery fry starter

Hap,

I understand yes, but I don't understand maybe. Could you elaborate, please?

Junkman

 

[Anonymous]

Maybe and "COULD" usually mean the same thing. I THEEENK 8O 8O

 

[argonsagas]

Maybe and "COULD" usually mean the same thing. I THEEENK 8O 8O

That is probably correct. :wink:



Every strand of wire used in the windings is insulated by a thin coating, a sort of non-conductive paint. Too much heat will embrittle the coating, causing it to break off. The resultant bared wires, especially when coupled with vibration, allow arcing, shorts, and it's over.

 

[earlfor]

NO, no Scotty. Maybe means it sometimes happens and could means sometimes it doesnt happen.

Earl

Maybe and "COULD" usually mean the same thing. I THEEENK 8O 8O

 

[Anonymous]

NO, no Scotty. Maybe means it sometimes happens and could means sometimes it doesnt happen.

Earl

Maybe and "COULD" usually mean the same thing. I THEEENK 8O 8O

?????????????OH???????????????? 8O 8O 8O

 

[Anonymous]

Ok, here is a short explanation with a link to a more thorough explanation. The quick answer is yes - a bad battery can cause premature starter motor failures. The reason being is that the starter motor develops a back emf which opposes current flow when it is turning properly. If the torque developed by the motor is not sufficient to overcome the load (engine resistance) then the motor will slow down. This reduces the back emf which then casues the current to increase which then causes excess heat to be produced in the armature windings. The heat is caused by the i*i*r (power) loss in the resistance of the wire in the armature. As you can see the current is squared when calculating the power loss - this is significant. Heat causes the breakdown of the insultion on the armature coils and you can all guess what happens next.

Here is a link that is quite good for those that want to explore this further.

http://www.mechatronics.me.vt.edu/book/Section3/motormodelling.html

 

[Hap Call]

Ok, here is a short explanation with a link to a more thorough explanation. The quick answer is yes - a bad battery can cause premature starter motor failures. The reason being is that the starter motor develops a back emf which opposes current flow when it is turning properly. If the torque developed by the motor is not sufficient to overcome the load (engine resistance) then the motor will slow down. This reduces the back emf which then casues the current to increase which then causes excess heat to be produced in the armature windings. The heat is caused by the i*i*r (power) loss in the resistance of the wire in the armature. As you can see the current is squared when calculating the power loss - this is significant. Heat causes the breakdown of the insultion on the armature coils and you can all guess what happens next.

Here is a link that is quite good for those that want to explore this further.

http://www.mechatronics.me.vt.edu/book/Section3/motormodelling.html

That was the most confusing paper I have read on PM motors, and I used to work for the company that supplied a lot of the info (Electro Craft, a part of Reliance Electric)! Back EMF is a very important part of DC motor design but in 99% of motor applications it is not needed for application design. Instead of using the P=I*I*R it is easier to understand by saying that power is equal to voltage times current:

P=I*V

and

V=I*R (Ohm's Law)

Substitute I*R for V (since they equal each other) in the power equation and you get

P=I*I*R

Which is a derived formula and really is not as useful in our application as the original power equation.

Okay, it takes the same amount of power (measured in either horsepower or watts) to turn an engine over no matter what voltage level you have. We will work in Watts in this example. If it takes 250 watts to turn the engine over to start it at 12 volts you get the following:

P=I*V

250 watts = I* 12V

Solving for I gives you 22.833 amps draw

Now look what you get when you reduce the voltage to 10V:

250 watts = I* 10V

Solving for I gives you 25 amps draw, or a 10% increase. What really screws you is the fact that you have a weaker spark so it is harder to start the motor, you end up grinding the starter longer, and thus you pull current through windings that were designed for a 2 second duty cycle for several seconds longer until it finally starts, if it does start. Also as motor pulls the current, the windings in the motor heat up increasing resistance and thus calling for even more current!

Sounds bleak doesn’t it!!! Well, check you battery often to keep the fluid levels up, keep your bike in good tune, and check your brushes once a year if you ride a lot. If you do that you should have little problem with your starter motor. They are a lot tougher then you would expect.

Hap

 

[Anonymous]

I hated to post something so technical because it tends to confuse people. You cannot solve for the current draw in a motor unless you know the back emf. Back emf is related to the armature angular velocity. This is not a strictly resistive circuit where r = v*i. I was trying to simplify the results so we didn't get into this type of discussion. If you were confused with that paper - don't feel bad. There are lots of electrical engineers who do not fully understand dc motors.

You say p=i*r. That is well and good for resistors. Capacitors, inductors and non-linear circuits have different governing equations. For example in an incuctor the equation v = L*didt or the voltage = the inductance * the time derivative of the current.

Let's just say that having a low battery voltage will cause the starter motor to draw more current which is bad.

I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves. I did this once before and had many people who didn't know didly criticize my post. Back emf is critical to understanding the current voltage relationship in a dc motor.

 

[Anonymous]

bad battery fry starter

Hap and Swanny,

I thank you both for your explanations Frankly I wouldn't have posed the question if I hadn't wanted a more thorough and technical explanation. An answer that is too simple may limit understanding. An answer that is too technical may bore the uninterested. An answer that has both will probably suit most.

Your explanations have made things a lot clearer. Thanks again.

 

[Hap Call]

I hated to post something so technical because it tends to confuse people. You cannot solve for the current draw in a motor unless you know the back emf. Back emf is related to the armature angular velocity. This is not a strictly resistive circuit where r = v*i. I was trying to simplify the results so we didn't get into this type of discussion. If you were confused with that paper - don't feel bad. There are lots of electrical engineers who do not fully understand dc motors.

You say p=i*r. That is well and good for resistors. Capacitors, inductors and non-linear circuits have different governing equations. For example in an incuctor the equation v = L*didt or the voltage = the inductance * the time derivative of the current.

Let's just say that having a low battery voltage will cause the starter motor to draw more current which is bad.

I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves. I did this once before and had many people who didn't know didly criticize my post. Back emf is critical to understanding the current voltage relationship in a dc motor.

Swanny, I do not want you to stop posting on technical issues...you raise many valid points. Back EMF is very critical in motor design and application and I really do understand it. I actually understand the paper you gave the link to BUT...when a paper opens with a cross product of two functions I believe it becomes too in-depth for most folks to understand. Most people have no idea that small omega (the little w looking symbol in the paper) is angular velocity normally measured in radians! I was trying to keep it simple enough for the layman to understand and by using straightforward Ohm's law and the power equation makes it a little more palatable.

Honestly, it is cool to hear about back EMF in an application other than those dealing with variable speed drive control and field loss functions. I hope you continue to post your explanations! I apologize if I slighted you in any way.

Oh yeah, one other thing… your explanation on back EMF was one of the best I have ever seen.

Junkman, sorry about the confusion. Swanny’s explanation is the most technically correct between his and mine. I was probably trying to keep it too simple.

Hap

 

[earlfor]

Well Swanny, I hope you'll reconsider. I found your answer very informative and interesting. It sure would become boring around here if everyone only posted answers that were common knowledge. There are many in the forums that work in specialized fields and could make posts such as yours. That is a GOOD thing.
Keep it up.

Earl

I don't think I will ever post anything technical about electronics here again as it just causes more problems than it solves.

 

[Anonymous]

bad battery fry battery

bad battery fry battery

The back emf factor is an interesting one. I am not an engineer but, I recall such a term from physics 101 many years ago. Sounds like in this application it would be emf induced by the coil passing through the field of the permanent magnet. I didn't realize it would play such a role in stabalizing current. Is the back emf in essence what keeps the armature from continuing to accelerate to infinity? If i*i*r=(heat generated from resistance of the wire), then any increase in current will cause the heat to increase by a factor of i raised to the power of 2 (an exponential increase, wow!!). I would think that TIME also plays a big role in how much heat is generated in the armature coil. When the armature slows down, the brushes have a longer contact time, thus allowing more heat build up in the coil.

Thanks for the discussion.